1. Field of the Invention
The present invention relates to a motor control device and, more particularly, to a control device of a motor acting to assist in driving a steering wheel of a vehicle.
2. Description of the Related Art
In the description hereinafter, an electric steering device that adds an assisting power to a steering device of a vehicle is taken as an example. However, the present invention is not limited to a steering device, but may be applied generally to a control device functioning to drive an electric motor with an inverter from a DC power supply.
As for an electric power steering device of vehicles, it is foreseen that the occurrence of error in the control device of a motor during the operation (generally an inverter is used) immediately invites dangerous situations, so that it is desirable to detect the error as soon as possible, for example, within one cycle of the inverter.
The following motor control device is disclosed in the Japanese Patent No. 2501030. With reference to FIGS. 1 and 3(b) of this Japanese Patent, a terminal voltage of a motor to be driven by an inverter is monitored and, further, with reference to FIGS. 1 and 3(a), a drive signal of switching element that drives the motor is monitored. In this motor control device, when the state of those inverter and switching element driving the motor continues for not less than a predetermined time period, an error is determined, and the feed of an electric power to the motor drive circuit is interrupted.
In the art of this Japanese Patent No. 251030, an output state only when a drive circuit makes outputs is monitored, so that a state of no output is not determined to be abnormal. However, the fault that a drive circuit cannot make outputs takes place relatively often as a mode of faults. Therefore, it cannot be said that this motor control device detects various faults with accuracy, thus a disadvantage exists in that this device is not always useful in practical use.
Moreover, in the case where a drive circuit continues to make outputs for a long time period takes place during actual driving, it is sometimes the case that a return torque is exerted on a steering wheel at all times, for example, as a result of running at high speed on a circular test course. To prevent the malfunction in such a case, it is necessary to set a time period on which an error is determined to be rather long. Consequently, a further disadvantage exists in that it takes a long time period to determine the error.
Furthermore, the motor control device disclosed in the mentioned Japanese Patent No. 2501030 shows only the case where a drive circuit is a single-phase inverter. Thus, a yet further disadvantage exists in that this drive circuit is not always effective in the case of a three-phase or poly-phase inverter.